Polishing pad for endpoint detection and related methods

ABSTRACT

A polishing pad has a polishing layer with a polishing surface and a back surface. A plurality of grooves are formed on the polishing surface, and an indentation is formed in the back surface of the polishing layer. A region on the polishing surface corresponding to the indentation in the back surface is free of grooves or has shallower grooves.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/444,921, filed May 23, 2003, which claims the benefit of U.S.Provisional Application Ser. No. 60/398,632, filed Jul. 24, 2002. U.S.application Ser. No. 10/444,921 is also a continuation-in-part of U.S.application Ser. No. 09/574,008, filed on May 19, 2000. U.S. applicationSer. No. 10/444,921 is also a continuation-in-part of U.S. applicationSer. No. 10/123,917, filed on Apr. 16, 2002, which claims the benefit ofU.S. Provisional Application Ser. No. 60/353,419, filed Feb. 6, 2002.The entirety of each of the above applications is incorporated herein byreference.

BACKGROUND

This present invention relates to polishing pads used in during chemicalmechanical polishing and methods and apparatus for monitoring apolishing process.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. One fabrication step involves depositing a fillerlayer over a non-planar surface, and planarizing the filler layer untilthe non-planar surface is exposed. For example, a conductive fillerlayer can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization is needed to planarize thesubstrate surface for photolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm leads to increased circuit resistance. On the other hand,under-polishing (removing too little) of a conductive layer leads toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

One way to determine the polishing endpoint is to monitor polishing ofthe substrate in-situ, e.g., with optical or electrical sensors. Onemonitoring technique is to induce an eddy current in the metal layerwith a magnetic field, and detect changes in the magnetic flux as themetal layer is removed. In brief, the magnetic flux generated by theeddy current is in opposite direction to the excitation flux lines. Thismagnetic flux is proportional to the eddy current, which is proportionalto the resistance of the metal layer, which is proportional to the layerthickness. Thus, a change in the metal layer thickness results in achange in the flux produced by the eddy current. This change in fluxinduces a change in current in the primary coil, which can be measuredas change in impedance. Consequently, a change in coil impedancereflects a change in the metal layer thickness.

SUMMARY

In one aspect, the invention is directed to a polishing pad. Thepolishing pad has a polishing layer having a front surface for polishingand a back surface. A first plurality of grooves are formed on the frontsurface of the polishing layer, and an indentation is formed in the backsurface of the polishing layer. A region on the polishing surfacecorresponding to the indentation in the back surface is either free ofgrooves or has a second plurality of grooves that are shallower than thefirst plurality of grooves.

Implementations of the invention may include one or more of thefollowing features. The region on the polishing surface corresponding tothe indentation may be substantially flat, e.g., it may free of grooves.Alternatively, the region on the polishing surface corresponding to theindentation may have the second plurality grooves. In addition, theregion may be opaque or transparent. The polishing layer may be aunitary structure. The recess may be formed in a second portion of thepolishing layer that is physically discrete from the first portion, andthe second portion may be secure to the first portion. The first andsecond portions may have substantially the same material composition,and the second portion may have a top surface substantially flush withthe polishing surface. An aperture may be formed in the first portion,and the second portion may be secured in the aperture. The secondportion may have a top section with a first cross-sectional dimensionand a bottom section with a second, different cross-sectional dimension.For example, the first cross-sectional dimension may be less than thesecond-cross-sectional dimension. The second plurality of grooves mayextend past an inner surface of the indentation.

The pad may have a backing layer disposed on the back surface of thepolishing layer. The backing layer may be softer than the polishinglayer. The backing layer may have an aperture therethrough, and theaperture may be aligned with the indentation in the back surface of thepolishing layer. The backing layer may be a thin non-compressible layer.The first plurality of grooves may be formed on a first portion of thepolishing layer, and the recess may be formed in a second portion of thepolishing layer that is physically discrete from the first portion. Asecond aperture may be formed in the polishing layer, and the secondportion may be secured in the second aperture. The first aperture mayhave first cross-sectional dimension and the second aperture may have asecond, different (e.g., larger or smaller) cross-sectional dimension.

In another aspect, the invention is directed to a polishing system. Thepolishing system has a carrier to hold a substrate, a polishing padsupported on the platen, and an eddy current monitoring system. Thepolishing pad includes a polishing layer having a front surface forpolishing and a back surface, a first plurality of grooves formed in thefront surface of the polishing layer, and an indentation formed in theback surface of the polishing layer. A region on the polishing surfacecorresponding to the indentation in the back surface is either free ofgrooves or has a second plurality of grooves that are shallower than thefirst plurality of grooves. The eddy current monitoring system has atleast one of a coil and a core extending at least partially into therecess in the back surface of the polishing layer to monitor a metallayer on the substrate held by the carrier.

In another aspect, the invention is directed to a method ofmanufacturing a polishing pad. The method includes forming a firstplurality of grooves in a polishing layer of the polishing pad, formingan indentation in a back surface of the polishing layer, and forming aregion on the polishing surface corresponding to the indentation that iseither free of grooves or has a second plurality of grooves that areshallower than the first plurality of grooves.

Implementations of the invention may include one or more of thefollowing features. The polishing layer may be secured to a backinglayer. Forming the recess may include machining the recess or moldingthe recess. Forming the indentation in the back surface may includesecuring a physically discrete first portion of the polishing pad havingthe indentation in an aperture in a second portion of the polishing padhaving the grooves.

In another aspect, the invention is directed to a method of polishing.In the method, a substrate is brought into contact with a front surfaceof a polishing layer of a polishing pad, the polishing layer having afirst plurality of grooves formed in a first portion of the frontsurface of the polishing layer and an indentation formed in a backsurface of the polishing layer. A region on the polishing surfacecorresponding to the indentation in the back surface is either free ofgrooves or has a second plurality of grooves that are shallower than thefirst plurality of grooves. A polishing liquid is supplied to the frontsurface of the polishing layer, and relative motion is created betweenthe substrate and the front surface.

Implementations of the invention may include one or more of thefollowing features. A metal layer on the substrate may be monitored withan eddy current monitoring system that has at least one of a coil and acore extending at least partially into the recess in the back surface ofthe polishing layer.

In another aspect, the invention is directed to a polishing pad with apolishing layer having a front surface and a back surface. The frontsurface has a first portion with a plurality of grooves and a secondportion that is substantially flat, and the back surface has a recessaligned with the second portion of the front surface.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view, partially cross-sectional, of achemical mechanical polishing station that includes an eddy currentmonitoring system.

FIG. 2 is a schematic top view illustrating the polishing pad of FIG. 1.

FIG. 3 is a schematic cross-sectional side view illustrating thepolishing pad of FIG. 2 along line 3-3.

FIG. 4 is a schematic cross-sectional side view illustrating a polishingpad having multiple indentations in the bottom surface of the coveringlayer.

FIG. 5 is schematic cross-sectional side view illustrating a polishingpad in which a grooveless insert is secured to a grooved polishing pad.

FIG. 6 is schematic cross-sectional side view of another implementationof a polishing pad in which the backing layer is a thin sheet.

FIGS. 7A and 7B are schematic cross-sectional side views of anotherimplementation of a polishing pad in which an insert is secured to abottom surface of the covering layer.

FIG. 8 is a schematic cross-sectional side view illustrating a polishingpad having shallow grooves over the recess.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, one or more substrates 14 can be polished at apolishing station 10 of a CMP apparatus. A description of a suitablepolishing apparatus can be found in U.S. Pat. No. 5,738,574, the entiredisclosure of which is incorporated herein by reference.

The polishing station 10 includes a rotatable platen 16 on which isplaced a polishing pad 18. The polishing pad 18 can be a two-layerpolishing pad with a soft backing layer 20 and a hard durable outerlayer 22 with a substantially uniform composition. The durable outerlayer 22 provides a polishing surface 24. At least a portion of thepolishing surface 24 can have grooves 28 for carrying slurry. Thepolishing station can also include a pad conditioner apparatus tomaintain the condition of the polishing pad so that it will effectivelypolish substrates.

During a polishing step, a slurry 30 containing a liquid and a pHadjuster can be supplied to the surface of polishing pad 18 by a slurrysupply port or combined slurry/rinse arm 32. Slurry 30 can also includeabrasive particles.

The substrate 10 is held against the polishing pad 18 by a carrier head34. The carrier head 34 is suspended from a support structure, such as acarousel, and is connected by a carrier drive shaft 36 to a carrier headrotation motor so that the carrier head can rotate about an axis 38.

A recess 40 is formed in platen 16, and an in-situ monitoring module 42fits into the recess 40. The in-situ monitoring module 42 can includesan situ eddy current monitoring system with a core 44 positioned in therecess 26 to rotate with the platen. Drive and sense coils 46 are woundthe core 44 and are connected to a controller 50. In operation, anoscillator energizes the drive coil to generate an oscillating magneticfield 48 that extends through the body of core 44. At least a portion ofmagnetic field 48 extends through the polishing pad 18 toward thesubstrate 12. If a metal layer is present on the substrate 10, theoscillating magnetic field 48 will generate eddy currents. The eddycurrent produces a magnetic flux in the opposite direction to theinduced field, and this magnetic flux induces a back current in theprimary or sense coil in a direction opposite to the drive current. Theresulting change in current can be measured as change in impedance ofthe coil. As the thickness of the metal layer changes, the resistance ofthe metal layer changes. Therefore, the strength of the eddy current andthe magnetic flux induced by eddy current also change, resulting in achange to the impedance of the primary coil. By monitoring thesechanges, e.g., by measuring the amplitude of the coil current or thephase of the coil current with respect to the phase of the driving coilcurrent, the eddy current sensor monitor can detect the change inthickness of the metal layer.

The drive system and sense system for the eddy current monitoring systemwill not be described in detail, as descriptions of suitable systems canbe found in U.S. patent application Ser. Nos. 09/574,008, 09/847,867,and 09/918,591, filed Feb. 16, 2000, May 2, 2001, and Jul. 27, 2001,respectively, the entire disclosures of which are incorporated byreference.

Various electrical components of the eddy-current monitoring systems canbe located on a printed circuit board in the controller 50. Thecontroller can include circuitry, such as a general purposemicroprocessor or an application-specific integrated circuit, to convertthe signals from the eddy current sensing system into digital data.

As previously noted, the monitoring system 42 includes a core 44positioned in the recess 26.

Referring to FIGS. 2 and 3, the covering layer 22 of the polishing pad18 includes one or more recesses or indentations 52 formed in the bottomsurface of the covering layer. These indentations create one or morethin sections 54 in the covering layer of the polishing pad. The core 44and/or coils 46 can extend into the indentations 52 so that they passpartially through the polishing pad. By positioning the core or coilsclose to the substrate, the spatial resolution of the eddy currentmonitoring system can be improved. These recesses 52 can extend throughat least 50% of the thickness of the covering layer 22, e.g., through75-80%. For example, in a polishing pad having an covering layer 22 thatis 100 mils thick, the recess 52 can have a depth D1 of about 80 mils,leaving the thin section 54 with a thickness of about 20 mils.

As previously mentioned, the covering layer 22 can also include aplurality of grooves 28 formed therein. The grooves may be of nearly anypattern, such as concentric circles, straight lines, spirals, and thelike. However, the grooves do not extend over the thin section 54 in thecovering layer 22. Thus, the polishing surface 24 of the polishing padincludes portions with and without grooves, and the indentation islocated in one of the portions without grooves. The grooves 28 can be atleast 10 mils deep, e.g., about 20 mils deep. The grooves 28 can extendthrough about 20-25% of the thickness of the covering layer 22. Forexample, in a polishing pad having an covering layer 22 that is 80 milsthick, the grooves 28 can have a depth D2 of about 20 mils. The groovescan be sufficiently deep that they extend to or past the plane definedby the inner surface 58 of the recess.

In addition, the backing layer 20, if present, includes one or moreapertures 56 positioned to provide access of the core 44 and/or coils 46to the indentations 52. Thus, the core 44 and/or coils 46 can alsoextend through the backing layer 20. As illustrated in FIG. 2, a singleaperture 52 can extend across all of the indentations 52. However, asillustrated in FIG. 4, in another implementation there is one aperture56 aligned with each recess 52. However, for some polishing operations,only a single-layer polishing pad is used, and there is not backinglayer.

Referring to FIGS. 1 and 4, when the polishing pad 18 is secured to theplaten, the thin section 54 fits over the recess 26 in the plate andover a portion of the core and/or coil that projects beyond the plane ofthe top surface of the platen 16. By positioning the core 42 closer tothe substrate, there is less spread of the magnetic fields, and spatialresolution can be improved. Assuming that the polishing pad is not beingused with an optical endpoint monitoring system, then the entirepolishing layer, including the portion over the recess, can be opaque.

In one implementation (shown in FIG. 3), the covering layer 22 can bemanufactured, e.g., by a molding process, with grooves and recessespreformed in the upper and lower surfaces of the covering layer,respectively. Thus, the cover layer 22, including the grooved portionand the thin section, can be a single unitary body. The covering layer22 can be manufactured by a molding process, e.g., by injection moldingor compression molding, so that the pad material cures or sets in moldwith indentations that form the grooves recess. Alternatively, thecovering layer 22 can be manufactured by a more conventional technique,e.g., by scything a thin sheet of pad material from a block. The groovesand recess can then be formed by machining or milling the top and bottomsurfaces of the covering layer, respectively. Once the covering layer 22has been manufactured, it can then be secured to the backing layer 20,e.g., with an adhesive, with the recess 52 in the covering layer 22aligned with the aperture 56 in the backing layer 20.

Alternatively, as shown in FIG. 5, the polishing pad can be manufacturedin two parts. For example, the main body 60 of the pad can bemanufactured with grooves 28 (either by molding or machining) Agrooveless insert 62 having the recess 52 in its bottom surface can bemanufactured separately. The main portion 60 and the insert 62 can beformed from the same material. An aperture 64 is cut in the main portion60 of the covering layer 22, and the insert 64 is secured in theaperture 64, e.g., by an adhesive that bonds the insert 64 to the uppersurface of the backing layer 20. The thickness D₄ of the insert 62 canbe equal to the thickness D₃ of the covering layer 22, so that the topsurface of the insert 62 is flush with respect to the polishing surface24, or the thickness D₄ of the insert 62 can be slightly less than thethickness D₃ of the covering layer 22, so that the top surface of theinsert 62 is slightly recessed with respect to the polishing surface 24.

In another implementation, illustrated in FIG. 6, the backing layer 20is a thin sheet of non-compressible, tear-resistant material, such asMylar (this implementation could be considered to function as asingle-layer polishing pad). The Mylar sheet can be applied to the backof the covering layer 22, and then the insert 62 can be placed into theaperture 64 in the covering layer 22 and adhesively secured to the topsurface of the Mylar sheet 20. A portion of the Mylar sheet is thenremoved to expose the recess 52.

In another implementation, illustrated in FIG. 7A, the insert 62 issecured to the underside of the covering layer 22. In thisimplementation, the insert 62 includes a narrow upper portion 70 thatfits into an aperture 72 in the covering layer 22, and a wide lowerportion 74 that fits into an aperture 76 in the backing layer 20. Thetop surface 78 of the wide portion 74 can be adhesively secured to thebottom surface 79 of the portion of the covering layer 22 that projectsbeyond the backing layer 20. The upper portion 70 can have the samethickness as the covering layer 22 so that the top surface of the insertis flush with the polishing surface 24, whereas the lower portion 74 canbe thinner than the backing layer 20 to provide a gap between the platenand the insert.

Referring to FIG. 7B, a two-part insert can also be secured to a singlelayer polishing pad. In this implementation, a two-part aperture 80 withan upper section 82 and a lower section 84 of different cross-sectionaldimensions is formed in the covering layer 22. Assuming that thecovering layer and insert have the same rigidity, the lower portion 74can have the same thickness as the lower section 84 of the aperture.

Referring to FIG. 8, in another implementation, the portion of thepolishing surface 24 corresponding to the recess 52, i.e., the thinsection 54, can have very shallow grooves 28 a, whereas the remainder ofthe polishing surface can have deep grooves 28 b. The deep grooves 28 bcan be at least 10 mils deep, e.g., about 20 mils deep. In contrast, theshallow grooves 28 a must have a depth that is less than (e.g., lessthan 25% of) the thickness of the thin section 54. For example, if thethin section 52 has a thickness of 20 mils, the shallow grooves 28 a canhave a depth of about 5 mils.

The eddy current monitoring system can be used in a variety of polishingsystems. Either the polishing pad, or the carrier head, or both can moveto provide relative motion between the polishing surface and thesubstrate. The polishing pad can be a circular (or some other shape) padsecured to the platen, a tape extending between supply and take-uprollers, or a continuous belt. The polishing pad can be affixed on aplaten, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there could be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad.

In addition, although terms of vertical positioning are used, it shouldbe understood that the polishing surface and substrate could be heldupside down, in a vertical orientation, or in some other orientation.

The eddy current monitoring system can include separate drive and sensecoils, or a single combined drive and sense coil. In a single coilsystem, both the oscillator and the sense capacitor (and other sensorcircuitry) are connected to the same coil.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A polishing pad, comprising: a polishing piecehaving a polishing surface, a bottom surface, and an aperture extendingfrom the polishing surface to the bottom surface, the polishing pieceincluding a polishing layer having the polishing surface and a backinglayer having the bottom surface, the polishing piece having a pluralityof grooves formed in the polishing surface, the grooves extendingpartially but not entirely through the polishing layer, wherein theaperture includes a first section through the polishing layer and asecond section through the backing layer, and wherein the first sectionis wider than the second section; and a solid light-transmissive windowsituated in the aperture and secured to the polishing piece, the windowhaving a lower surface and an upper surface, the upper surfacesubstantially coplanar with the polishing surface and lacking grooves,the lower surface having an indentation that extends partially but notentirely through the window, wherein the indentation is empty, andwherein the window is configured such that the upper surface issubstantially fixed relative to the polishing surface.
 2. The polishingpad of claim 1, wherein a portion of the bottom surface at the peripheryof the window rests on a portion of the backing layer surrounding thesecond section of the aperture.
 3. The polishing pad of claim 1, whereinthe window is secured to the backing layer with an adhesive.
 4. Thepolishing pad of claim 1, wherein the lower surface of the window issubstantially coplanar with an underside of the polishing layer.
 5. Thepolishing pad of claim 1, wherein the indentation is a firstindentation, and the window includes a second indentation in the lowersurface.
 6. The polishing pad of claim 1, wherein the polishing layer isa unitary structure.
 7. The polishing pad of claim 1, wherein theplurality of grooves extend past an inner surface of the indentation. 8.The polishing pad of claim 1, wherein the indentation has a depth of atleast 50% of a thickness of the polishing layer.
 9. The polishing pad ofclaim 8, wherein the depth of the indentation is 75-80% of the thicknessof the polishing layer.
 10. The polishing pad of claim 8, wherein theplurality of grooves extend through about 20-25% of the thickness of thepolishing layer.
 11. The polishing pad of claim 1, wherein the backinglayer is softer than the polishing layer.
 12. The polishing pad of claim1, wherein the backing layer is non-compressible.